Metal strip, when deformed by rolling to reduce the thickness to a desired gauge, generates considerable heat, a portion of which is imparted into the work rolls performing the work. It is necessary that heat in the work rolls be controlled to a proper level to allow thermal shaping of the rolls, but not to an excessive amount where thermal instability occurs.
Typically a rolling mill is equipped with a coolant spray system which has a top work roll spray bar and a bottom work roll spray bar, as well as top and bottom back-up roll spray bars where back-up rolls are utilized. Each spray bar is a fixed header, often with some latitude for angular adjustment while out of operation, and is equipped with spray valves having nozzles directed to a fixed portion of the roll. Spray patterns are normally fixed, with coolant flow controlled by manually or solenoid operated valves. The area of impingement of each spray on the work roll is typically about 2 inches to 4 inches wide along the length of the work roll. In the case of the back-up roll spray bars, this zone width is adequate since the back-up roll spray bars, this zone width is adequate since the back-up rolls have less effect on strip shape than the work rolls. In the case of the work rolls, "tight edges" occur in the strip if the rolls in the areas at the edges of the strip are under cooled or over cooled relative to the remainder of the roll surface in contact with the strip. "Tight edges" refer to thickened strip margins and are caused by a temperature gradient that exists across the area of the work roll surface inboard the edge of the strip, where the roll is relatively hot, to that outboard the edge of the strip, where the roll is relatively cool. It is desirable to roll metal which is free of tight edges to improve shape and to allow faster rolling speeds. Adjustment of the coolant volume of the spray valves is inadequate for this purpose and hence there has been a long standing need for a method of preventing tight edges from occurring.